Electron discharge tube high-frequency amplifier and modulating circuits therefor



V. J. COOPER ET AL 2,472,195 HARGE TUBE HIGH-FREQUENCY AMPLIFIER ELECTRON DISC AND MODULATING CIRCUITS THEREFOR 2 Sheets-Sheet 1 June 7, 1949.

Filed Oct. 24, 1946 BE [AT/TEE June 7, 1949.

V. ELECTRON DISCHARGE TUBE HIGH-FREQUENCY AMPLIFIER Filed Oct. 24, 1946 J. COOPER ETAL 2,472,195

AND MODULATING CIRCUITS THEREFOR '2 Sheets-Sheet 2 mam/Mme INVENTOR5 wcm/za: more/z BY l GEEE/V ,44 ATTORNEY Patented June 7, 1949 UNITED STATES PATENT OFFICE ELECTRON DISCHARGE TUBE HIGH-FRE- QUENCY AMPLIFIER AND MODULATING CIRCUITS THEREFOR Delaware Application October 24, 1946, Serial No. 705,264 In Great Britain December 5, 1945 4 Claims. 1

The invention relates to electron discharge tube high frequency amplifiers and modulating circuits therefor.

A dimculty which commonly arises in radio frequency amplifiers of the grid modulated type is that the grid current taken by the amplifier tube or tubes is not a rectilinear function of the input and since this current constitutes a load upon the radio frequency excitation stage drivin th amplifier, the amplitude of excitation is likely to vary with variation of rid current consequent upon variation of modulating voltage. A known expedient for avoiding or reducing this disadvantage is to provide additional grid loading resistance for the driver stage so as to reduce the percentage load variation caused by grid current variation. This expedient, however, is wasteful of power imposing an extra load upon the driving stage.

The object of the present invention is to avoid or reduce the above mentioned disadvantage without incurring the substantial power wastage of the known expedient just described.

According to this invention a radio frequency amplifier is excited from a radio frequency exciter source through at least one stage to which said amplifier is in so-called cathode follower coupling relation, the said stage being thus arranged with respect to the amplifier as though it were a very low resistance generator whereby th defect above mentioned is reduced or avoided.

Modulating input may be applied in any convenient manner known per se to the driven radio frequency amplifier. Owing, however, to the well known inherent wide-band properties of the cathode follower type of connection, the invention lends itself to the application of modulation input at the point of lower power level constituted by the input to th stage to which the radio frequency amplifier is cathode-follower-coupled with obvious advantages as regards loading of the modulator.

The invention is illustrated in the accompanying drawings which show diagrammatically several embodiments.

Referring to Fig. 1, a radio frequency exciter source is connected between the control grids GiGz of a cathode follower stage comprising tubes VlVZ, the usual radio frequency tuned circuit L1C1 with earthed centre point P being connected between the grids. Each grid GiGz is cross-connected to the cathode K1 or K2 respectively of the other tube through a neutralising condenser Cs or C4 respectively, which balance out grid-cathode capacities, and the two cathodes are connected together through a tuned circuit CzLz constituting the cathode leg impedance. The anodes AIAZ are earthed through condensers AC1AC2 and receive anode potential from a common source (not shown). Voltage developed between the ends of the cathode leg impedance C2L2 is applied to the succeeding driven radio frequency amplifier stage which may be arranged in various different ways. For example, as shown in Fig. 1, it comprises two tubes V3V4 with their cathodes K3K4 earthed and their grids G'3G4 capacity-coupled by condensers C5C6 each to one end of the cathode leg impedance C2L2. Modulator output from a suitable modulator is applied to the junction point of two chokes L314 connected between the grids G3G4. Output to a load is taken from across an output tuned circuit C9L5 connected between the anodes A3A4, anode potential being applied through the midpoint of coil L5 from a potential source (not shown). Neutralisation of the grid-anode capacities of the driven stage amplifier tubes V3V4 is effected by cross-connecting the tube grids G3G4 through neutralizing condensers C'ZCs to the anodes A4A3.

In the modification shown in Fig. 2, the amplifier tube grids GaG4 are connected to opposite ends of a neutralising impedance N having a center tap CT to which th modulator is connected, the cathodes being connected together through chokes 143114 in positions vis-a-vis the preceding stage, corresponding to those of chokes L3L4 of Fig. 1. Thus it will be seen that Fig. 2 incorporates a grounded grid, grid modulated amplifier.

Fig. 3 shows a third embodiment which differs from Fig. 1 in that the output tuned circuit CsLs from which the load is fed is connected between the cathodes K3K4 instead of between the anodes A3A4, the neutralising condensers C'zCa being in this case cross-connected between the grids G3G4 and the cathodes K4K3 of the tubes V3V-i. Thus the amplifier is also in cathode follower coupling relation with the load.

If desired, modulation may be eiiected at the input circuit of the stage preceding the amplifier. Thus the embodiment illustrated in Fig. 4 differs from Fig. 1 only in that the modulator is connected to the center tap P on the coil of the circuit L1C1 between the grids GIGZ of the tubes V1V2. Similarly, the embodiment of Fig. 5 resem'bies that of Fig. 2, the only difierence being that the modulator is connected as in Fig. 4. Again, the embodiment of Fig. 6 difiers from that of Fig. 3 only in that the modulator connection is 3 taken to the mid-point of the grid circuit of the tubes V1Vz.

The invention is not limited to the precise arrangements shown for example. Although, for simplicity in drawing, triodes have been illustrated, other forms of tubes, e. g. pentodes, may,

be employed as may be desired.

What is claimed is:

1. In an amplifier system; a s'ourceof currents of carrier wave frequency to be amplified, a pair of electron discharge tubes each having electrodes 1 including an anode, a cathode and'control grid,-

a parallel tuned circuit coupled-to said source and connecting the control grids of said tubes in:

push-pull relation, couplings of low impedance to said currents of carrier wave frequency coupling. the anodes of said tubes to ground, a second parallel resonant circuit coupled between the cathodes of said tubes, neutralizing capacitors cross coupling the control grids and cathodes of said::tubes, a second pair of tubes each having electrodes'including an anode, a cathode and a control grid, anoutput circuit coupled between the: anodes of said last named pair of tubes, connections coupling. said last named parallel resonant circuit in push-pull relation to corresponding electrodes of said last named pair of tubes, a-lmodulator circuit coupling the control grids of =said.last named pair of tubes in parallel, and neutralizingjapparatus connected with the control grid of each of said last named tubes.

2. Anhamplifier system as recited in claim 1 wherein said corresponding electrodes are the controlgrids of: said second named pair of tubes.

3.: An amplifier system as recited in claim'l wherein said corresponding electrodesare the cathodesof said second named pair of tubes.

4. In an amplifier system, a source of currents of carrier wave frequency to be amplified, a pair of electrode structures each including an anode, acathoderand ascontrol grid, a parallel resonant circuit coupled to said 'source and connecting said control grids in push-pull relation, couplings of low impedance to said currents of carrier wave frequency couplingr" said anodes to ground, a secondzparallelfresonant circuit coupled between said icathodes-,xneutralizin capacitors cross coupling said control grids and cathodes, a second pair. offlele'ctrodel; structures each including an anodamaicathode and a control grid, an output circuit coupled-between the anodes of said second pair of structurea'aconnections coupling said secondparallelresonant circuit in push-pull relation toscorresponding electrodes of said second pair of structures, a modulator circuit coupling the controlngrids of said second pair of structures in parallel, and neutralizing apparatus connected with -thecontrol. grid of each of saidiasecond structure.

VICTOR JAMES COOPER. ERNEST C-rRIZIEN;v

REFERENCES. CITED- The follow-ingvreferences'are of record in the file-of: thisxpatent:

UNITED STATES PATENTS Number' Name Date 2,136,479 Trevor Nov. 15, 1938 2,161,844 B'obler June 13, 1939 2,255,882 Hathaway et'al.' Sept. 16, 1941 2,326,314 Usselman' Aug. 10, 1943 2,330,109= Brown Sept. 21, 1943 

